Install toplevel handler inside fork.

When rts is forked it doesn't update toplevel handler, so UserInterrupt
exception is sent to Thread1 that doesn't exist in forked process.

We install toplevel handler when fork so signal will be delivered to the
new main thread.

Fixes #12903

Reviewers: simonmar, austin, erikd, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2770

GHC Trac Issues: #12903

(cherry picked from commit 895a131f)